What is Knob and Tube Wiring?
Knob and tube (K&T) wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consists of insulated copper conductors run through ceramic knobs mounted to framing members and through porcelain tube insulators between locations where conductors entered boxes or devices.
Some key characteristics of K&T wiring:
- Uses single insulated wires, rather than bundled cables
- Wires are separated by air, rather than insulation
- Wires are supported along their length, reducing stress
- Insulators resist moisture and provide protection against contact
While K&T was a safe and effective wiring method for its time, it lacks many modern safety features and is not designed to handle the power demands of today's homes.
Dangers of Retaining Knob and Tube Wiring
There are several important reasons why K&T wiring should be properly updated in homes:
- Fire hazard - insulation on old wires can crack or deteriorate over time, exposing copper that could arc and start fires. K&T lacks protective sheathing.
- Insufficient grounding - K&T systems often have no equipment grounding conductor, increasing risk of shocks.
- Overload - designed for lower electrical loads, K&T can overheat from modern demand. Its max capacity is 60A.
- Unsafe splices - DIY connections and taps added over the years may not be secure.
- Insurance issues - many providers won't insure homes with K&T or charge significantly higher premiums.
While problems aren't guaranteed, the risks and limitations make retirement or upgrading advisable. Never assume old wiring is safe.
Options for Upgrading Knob and Tube Wiring
Full K&T replacement - The most thorough option is to remove all K&T wiring and replace it with modern NM cable meeting current code requirements. This ensures maximum safety and full electrical capacity but is also the most invasive and costly upgrade.
Partial replacement - Strategically replacing K&T wiring in high risk areas can provide substantial safety improvements without the cost of a full rewire. Prioritize high load circuits or wiring showing degradation.
Overhaul plus new panel - An alternative is leaving K&T intact but disconnecting it from use. Install a new electrical panel wired with modern cabling just for new circuits. This offsets the original system's limitations.
Circuit breaker upgrades - Upgrading fuse boxes to modern circuit breakers adds capacity limits absent in original K&T installations. Doesn't address other risks but is a cheap, non-invasive measure.
Insulation - Blowing insulation into wall cavities can be risky with K&T present. Safer options include foam injection or insulating just the exterior wall surface.
Best Practices for Safe K&T Retrofits
For any upgrades or alterations to live K&T wiring, extreme care should be taken not to disturb or stress the old, fragile insulation:
- Turn off power at the main panel before starting work, then verify it's off via a non-contact voltage tester. Leave it off until all work is completed.
- Avoid drilling or nailing into areas where K&T wiring could be hidden. Use care when fishing new cables.
- Don't use K&T circuits to power heavy, high-draw appliances like dryers, AC units, and electric heaters.
- Install GFCI outlets on K&T circuits to add protection from shocks. Test them regularly.
- Address any improper splices, double-taps, or loose connections found. Use wire nuts and junction boxes.
- Label all K&T circuits in the electrical panel so they can be easily identified.
- Consult an electrician if replacement wiring is beyond your skill level or comfort.
With the right precautions, K&T wiring can be made safer, but full replacement is the best permanent solution. Prioritize critical needs and upgrades within your budget to make sensible improvements over time.